Luminescence of Eu³⁺ in GaN(Mg, Eu): transitions from the ⁵D₁ level

Eu-doped GaN(Mg) exemplifies hysteretic photochromic switching between two configurations, Eu0 and Eu1(Mg), of the same photoluminescent defect. Using above bandgap excitation, we studied the temperature dependence of photoluminescence (TDPL) of transitions from the excited ⁵D₁ level of Eu³⁺ for both configurations of this defect. During sample cooling, ⁵D₁→⁷F_0,1,2 transitions of Eu0 manifest themselves at temperatures below ~200 K, while those of Eu1(Mg) appear only during switching. The observed line positions verify crystal field energies of the ⁷F_0,1,2 levels. TDPL profiles of ⁵D₁→⁷F₁ and ⁵D₀→⁷F_J transitions of Eu0 show an onset of observable emission from the ⁵D₁ level coincident with the previously observed, but hitherto unexplained, decrease in the intensity of its ⁵D₀→⁷F_J emission on cooling below 200 K. Hence the ⁵D₀→⁷F_J TDPL anomaly signals a back-up of ⁵D₁ population due to a reduction in phonon-assisted relaxation between ⁵D₁ and ⁵D₀ levels at lower temperatures. We discuss this surprising result in the light of temperature-dependent transient luminescence measurements of Eu0.